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FIG. 2.—Compression temperatures..
T<i — Ti-\/(pz/p1)Q-2g for two-stage compression, and by T2 = ^i^Xfe/Pi)0-29 for three-stage. Values of Ti may be obtained readily from the chart devised by F. W. O'Neill and shown in 2, in which the ordinates give the factors by which the initial
absolute temperature TI (degree Fahrenheit) is to be multiplied to obtain the final absolute temperature TZ for various ratios of p2 to pi.
Fans.—As said above, these are used only at low pressures. They are of two types, the propeller and the centrifugal. The former moves the air in a direction parallel to its axis of rotation, the effect being similar to that of a ship's propeller. The centrifugal fans have an inlet at the center and discharge at the periphery.
Fans may operate with equal efficiency at any speed which their mechanical strength safely allows. They may also discharge, in practice, through orifices varying in area from that of their full discharge, to a very small per cent of the full outlet. The efficiency varies with different orifice areas, but not according to any fixed law for all designs.
The horsepower for any fan or blower, if the area of the discharge orifice remains unchanged, varies directly as the cube of the speed; also, if the area of the discharge orifice remains unchanged and the speed is constant, the horsepower varies directly with the density of the gas being handled.
The horsepower of a centrifugal fan, if the speed remains unchanged, generally decreases as the area of discharge orifice is decreased. In some designs the horsepower with rated orifice is not over one-third of the horsepower with orifice equal to area of fan outlet; and in other designs it is very little less at rated orifice than with unrestricted orifice.
The horsepower of a propeller fan, if the speed remains unchanged, increases as the area of the discharge orifice is decreased. In some designs the horsepower with rated orifice is not over one-quarter of the horsepower with complete restriction.
The propeller fan is used in heating, ventilating, cooling, drying, etc., rarely for pressures exceeding 1 in. of water Their rated capacities are given in the accompanying table.
Wheel	Free delivery			Static pressure, inches						1.0		
				0.25			0.5					
diameter,	||	O 3	1	||	O  3	cu	«!	0 3	fcn	? 3	O   3	g
feet	«*     d	£.3	jS o	<+H    P		^g	•^.9	'•5 2	JS o	£d	'43 d	'tf  O
	0  g			° a		cS  ft			OS  g.			
	3 JH	g	PQ §		"o q >    (H	PQ «	X!	g ^	W >S	<pS   M	g	pq   OT
		# a	0	o ft	«ss,	o	o g,	(5 a	o	O a	f£ a	O -a
2	4,000	170	0.24	2,100	710	0. 53	3,000	1,000	1.5	4,250	1,420	4.2
3	9,000	465	0.54	4,800	470	1.2	6,750	670	3.4	9 , 500	940	9.0
4	16,000	350	0.95	8,500	355	2. 1	12,000	500	G.O	17,000	710	17.0
6	36,000	233	2.15	19,000	235	5. 1	27,000	335	14.5	38 , 000	470	41.0
8	64,000	175	3.8	34 , 000	175	8.5	48 , 000	250	24.0	68,000	355	68. 0
Approximate   Horsepower   of Propeller Fans.—The  horsepower  required to drive any propeller fan may be represented by an expression of the form,